AC power line filter

ABSTRACT

A parallel resonant circuit for removing noise and harmonic frequencies from the AC power source is connected directly in parallel to a power source with no intervening electrical components. The parallel resonant circuit is comprised of at least one inductor for drawing an inductive current that is substantially equal to but one hundred and eight degrees out of phase with at least one capacitor that draws a capacitive current. The capacitors and the inductors of the parallel resonant circuit are connected in parallel and may be tuned to the fundamental frequency of the power line.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a Continuation-In-Part (CIP) ofco-pending U.S. patent application Ser. No. 10/155,161, filed May 24,2002, pending, the entire disclosure of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention generally relates to filtering circuitscomprised of a parallel resonant circuit, and more particularly, toparallel resonant circuits that directly connect to AC power sourceswithout any intervening elements, reducing all frequency distortions inalternative currents, including harmonic distortion.

[0004] 2. Description of Related Art

[0005] In general, as illustrated in the prior art FIG. 1A, parallelresonant circuits 6 comprised of a capacitor 12 connected in parallel toan inductor 10 are always connected to a power source 2 through one ormore electrical components 8. Each of the components 8A and 8B may forexample comprise of one or more inductors to isolate a load 4 from asource 2, one or more resistors to dampen oscillations or dissipatepower, or some other elements to perform other functions. The components8 do not represent inherent or intrinsic characteristics of anyelectrical component, but represent extrinsic, additional componentssuch as actual resistors or inductors. The circuit topography comprisedof the parallel resonant circuits 6 coupled with at least one or moreother components 8 is purported to reduce harmonic distortions in analternative current waveform, in addition to the functions describedabove, with the additional functions depending on the type(s) ofelement(s) 8 always connected to the parallel resonant circuit 6.

[0006] When AC current flows through the inductance 10 a backelectromotive force (emf) or voltage develops across it, opposing anychange in the initial AC current. This opposition or impedance to changein current flow is measured in terms of inductive reactance. Theinductive reactance is determined by the formula:

Z _(L)=(2πfL)  (1)

[0007] Where

[0008] f=Operating Frequency

[0009] L=Inductance

[0010] Z_(L)=Reactive Impedance of the Inductor.

[0011] When AC voltage develops across the capacitor 12, an opposingchange in the initial voltage occurs, this opposition or impedance to achange in voltage is measured in terms of capacitive reactance. Thecapacitive reactance is determined by the formula:

Z c=1/(2πfC)  (2)

[0012] Where

[0013] f=Operating Frequency

[0014] C=Capacitance p1 Z_(C)=Reactive Impedance of the Capacitor.

[0015] Resonance for circuit 6 occurs when the reactance Z_(L) of theinductor 10 balances the reactance Z_(C) of the capacitor 12 at somegiven frequency f. The resonance frequency is therefore determined bysetting the two reactance equal to one another and solving for thefrequency, f.

(2πfL)=1/(2 90 fC)  (3)

[0016] This leads to:

f _(RE)=1/2π{square root}LC  (4)

[0017] Where

[0018] f_(RE)=Resonant Frequency.

[0019] In general, the parallel resonant circuits present very highimpedance to those electrical signals that also operate at the sameresonant frequency, f_(RE). At resonance, input signals with frequenciesbecoming far removed from the resonance frequency f_(RE) seeever-decreasing impedance presented by the parallel resonant circuit.For example, if parallel resonant circuit 6 illustrated in FIG. 1A istuned to resonate at the fundamental frequency of the power source 2,where f_(RE)=f_(FUND), the input current signals from power source 2that operate at frequencies equal to f_(FUND) will be rejected bycircuit 6 and will pass onto the load 4. To these current signals, theparallel resonant circuit 6 is almost invisible because it behavesalmost like an “open circuit” at f_(RE)=f_(FUND). As the input currentsignals depart from the resonant frequency, up or down, the parallelcircuit 6 presents a lessening impedance and progressively allows othersignals (those not operating at f_(RE)) to leak to ground. For signalsat frequencies far removed from resonance, the parallel resonant circuit6 presents a short path to ground. Using these principles, parallelresonant circuits 6 may be tuned to the fundamental frequencies of thepower source 2 to therefore filter out frequencies above or below thefundamental, providing low noise signals to load 4. The filtering actionis mainly done by the capacitance portion of the parallel resonantcircuit, with the inductance part “giving back” the capacitive currentdrawn by the capacitor. In general, one may look at the impedancepresented by the parallel resonant circuit in terms of its capacitiveimpedance Z_(C) of equation (2) above. Accordingly, for high frequenciesthe denominator of equation (2) having the frequency value f willincrease, making the total impedance of the parallel resonant circuitsmaller.

[0020] The amount of noise on signals passed on to load 4 depend mostlyon how much of lessening impedance any path to ground presents for inputsignal with operating frequency above or below the desired operatingfrequency. In particular, the total impedance of any path to ground mustbe considered to determine the appropriate filtering effect for signalswith undesirable frequencies, and not just that of the parallel resonantcircuit. In the instance of FIG. 1A, the total impedance includes thatpresented by the parallel resonant circuit 6 and those of any component8 coupled thereto. Therefore, the total impedance of a path to groundfor signals with undesirable operating frequency will not behave as ashorted path even if the parallel resonant circuit behaves ideally andpresents a “short circuit” behavior. Components 8 will still maintainand present impedance commensurate with their rated values, regardlessof any frequency variations. Accordingly, the true impedance of thecircuit path to ground for the combination of the parallel resonantcircuit 6 and the components 8 is given by:

Z _(TOTAL) =Z _(PRC) +Z ₈  (5)

[0021] Where

[0022] Z₈=Impedance of elements 8A or 8B.

[0023] Z_(PRC)=Impedance of the Parallel Resonant Circuit

[0024] Z_(TOTAL)=Total impedance.

[0025]FIG. 1B graphically illustrates the consequence of the additionalimpedance Z₈ of component(s) 8. As shown, as the frequency f increases(moves away from the resonant frequency), the total impedance Z_(TOTAL)illustrated by line 14 decreases, allowing short path for currentsignals with undesirable frequencies to ground, filtering these signals.However, even if the frequencies become very large where the Z_(PRC) ofthe Z_(TOTAL) becomes almost zero, Z_(TOTAL) itself will than equal toZ₈. Hence, for frequencies much higher than those desired, equation (5)will equal:

Z _(TOTAL)=0+Z ₈  (6)

[0026] Z_(TOTAL) can never present a short circuit path for signals withfrequencies removed from the desired operating frequency due toimpedance of one or both of the elements 8A and 8B. Hence, all theundesirable frequencies illustrated in region 16 of the graph willcontinue to be passed on to the load 4, regardless of how low of animpedance the parallel resonant circuit 6 presents to the signals thatoperate away from the resonant frequency.

[0027] As a specific example, U.S. Pat. Nos. 5,323,304 and 5,570,006,both to Woodworth, the entire disclosures of which are incorporatedherein by reference, teach in their respective FIG. 1 the use ofparallel resonant circuit 20 coupled through an inductor 21 to a powersource 12. In this instance, the inductor 21 would constitute theelements 8A of the prior art FIG. 1A of the present invention. As taughtin Woodworth, the series connected inductor 21 isolates the power source12 from the load 16 such that harmonic currents that may be generated bythe load 16 will minimally affect the power source 12. In addition, theinductor 21 also serves to increase the effective impedance of the powersource 12 as seen by the load 16, limiting the amount of power that canbe drawn by the load. This increase in effective impedance (Z₈ of theinductor 21) degrades the filtering effect of the parallel resonantcircuit, and as illustrated in prior art FIGS. 2A, distorts the outputcurrent and voltage supplied to a load.

[0028] U.S. Pat. No. 3,237,089 to Dubin et al shows a similar circuitwhere inductor L_(s) is connected in series with the parallel resonantcircuit LC, comprised of an inductor L connected in parallel with acapacitor C. The circuit topography illustrated is a simplifiedequivalent circuit of a saturable-type constant voltage transformers,where inductor L_(s) isolates the power source e_(i) from a load. Thiscircuit is illustrated only for as a way to show how a constant voltagetransformer functions. Therefore, the reference U.S. Pat. No. 3,237,089is only concerned with voltage level control, and not filtering action.

[0029] Many electronic devices (loads) today draw current only at thepeaks of the sinusoidal AC power supply voltage. This cause the peaks ofthe AC supply waveform to become flattened out because of thisnon-linear loading of the power grid, reducing the amount of powersupply required by loads. As illustrated in the prior art FIG. 2A, thisis easily detected by measuring the amount of current I_(L) 20 drawn byload 4 of FIG. 2B, and the sinusoidal voltage V_(L) 18 across the load4. The current drawn by the load 4 at the peak of the sinusoidal voltagecauses the voltage waveform 18 to be flattened at its sinusoidal peak.The more loads are connected to a power source, the flatter the waveformof the voltage across those loads.

[0030] Adding components 8 (FIG. 1A) exasperate the above-describedsituation, worsening the flattening of the voltage waveform at the load.For example, the sudden draw of current 20 by load 4 at the peak of thevoltage 18 produces an opposing voltage across inductor 24 (due to theinductive reactance), lowering even further the peak of the voltage 18available to the load 4. Addition of components 8 distorts the voltagewaveform 18 across the load 4, generating noise thereat. Noise isgenerated because load requirements for appropriate load current andvoltage are not met. Hence, even low value inductors 24 in series withthe power source 2 and the parallel resonant circuit 6 cause muchtrouble.

[0031] As another specific example, the U.S. Pat. No. 5,343,381 toBolduc et al, the entire disclosure of which is incorporated herein byreference, teach in their FIG. 1 the use of a resistor element 8connected in series with a parallel resonant circuit that is comprisedof a capacitor 4 connected in parallel with an inductor 6 to produce adampening circuit 2. In this instance, the resistor 8 of Bolduc et alwould constitute the elements 8B of the prior art FIG. 1A of the presentinvention. The dampening resistor 8 degrades correction of any possibleoutput distortions illustrated in prior art FIG. 2A of the presentinvention. In addition, the LC filtering effect is also degraded due tothe added impedance of resistor 8, as graphically illustrated in theprior art FIG. 1B of the present invention. In this instance, theimpedance Z₈ illustrated in FIG. 1B equal the value of resistor 8.

[0032] As described and illustrated, parallel resonant circuits havealways been connected to a power source through some other componentthat degrades or negates the resonant circuit's performance in terms ofoutput signal correction and filtering of signals that operate atundesired frequencies.

BRIEF SUMMARY OF THE INVENTION

[0033] The present invention provides a simple and novel circuittopography for correcting output signal distortions and filteringsignals operating at undesirable frequencies using a parallel resonantcircuit that connects directly to a power source with no interveningcomponents between the source and the parallel resonant circuit.

[0034] By removing intervening components from between the power sourceand the parallel resonant circuit, the impedance of those components isalso removed. Accordingly, at resonance, input signals with frequenciesbecoming far removed from the resonance frequency will only see an everdecreasing impedance presented by the parallel resonant circuits with noother electrical components to present additional impedance that degradeor negate the performance of the parallel resonant circuit. In addition,the circuit topography of the present invention improves the restorationof output signal distortions that are generally caused and exacerbatedby the addition of electrical components.

[0035] The direct connection of resonant circuit to a power sourcecorrects voltage and current distortions in a power system operating ata system line frequency wherein the resonant circuit is directlyconnected in parallel with a source, with no intervening components. Theresonant circuit includes at least one capacitor for drawing acapacitive current and at least one inductor for drawing an inductivecurrent equal in amplitude and opposite in phase with the capacitivecurrent. The at least one inductor is connected in parallel with thecapacitor to form a parallel resonant circuit. The resulting parallelresonant circuit is tuned to resonate at the system line frequency suchthat the parallel resonant reactance of the circuit is at its peak atthe system line frequency and lower at frequencies above and below thesystem line frequency As such, the parallel resonant circuit absorbsvoltage perturbations in excess of the amplitude of the power systemsignal at all frequencies above or below the system line frequency andprovides energy to restore notches in the amplitude of the power systemsignal at all frequencies above or below the system line frequency.

[0036] The present invention is also directed to a method for correctingvoltage and current distortions in a power system operating at a systemline frequency comprising the steps of forming a parallel resonantcircuit wherein the circuit comprises at least one capacitor for drawinga capacitive current and at least one inductor for drawing an inductivecurrent equal in amplitude and one hundred eighty degrees out of phasewith the capacitive current connected in parallel with the capacitor,wherein the parallel resonant circuit is tuned to resonate at the systemline frequency. The method further comprises the step of connecting theparallel resonant circuit in parallel with a power source with nointervening components between the power source and the parallelresonant circuit.

[0037] Accordingly, the addition of a device constructed according tothe present invention greatly diminishes the effective power lineimpedance as seen by the load at frequencies above and below thesystem's power line frequency and thereby limits any local distortion atthe load. The impedance at the output terminals of the device is verylow and may source current at frequencies both above and below that ofthe power line. The parallel impedance of the power line and thedevice(s) connected to it provide impedance far less than eitherimpedance alone. This lower source impedance offers the load a stifferpower source that does not sag or drop out during high loadingconditions due to load turn-on and turn-off impulses.

[0038] These and other features, aspects, and advantages of theinvention will be apparent to those skilled in the art from thefollowing detailed description of preferred non-limiting embodiments,taken together with the drawings and the claims that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] It is to be understood that the drawings are to be used for thepurposes of exemplary illustration only and not as a definition of thelimits of the invention.

[0040] Referring to the drawings in which like reference numbers presentcorresponding parts throughout:

[0041]FIG. 1A is a prior art exemplary illustration of circuittopography used with a parallel resonant circuit;

[0042]FIG. 1B is a prior art exemplary graphical illustration offrequencies not filtered out by the circuit of FIG. 1A;

[0043]FIG. 2A is a prior art exemplary graphical illustration of avoltage across and the current through a load;

[0044]FIG. 2B is a prior art schematic illustration of an exemplarycircuit with a series connected inductor coupled to a resonant circuit;

[0045]FIG. 3A is an exemplary graphical illustration of a voltage acrossand the current through a load in accordance with the present invention;

[0046]FIG. 3B schematically illustrates an exemplary power system usinga parallel resonant circuit directly connected to a power source inaccordance with the present invention;

[0047]FIG. 3C schematically illustrates an exemplary graphicalillustration of impedance vs. frequency for the circuit of FIG. 3B inaccordance with the present invention;

[0048]FIG. 4A schematically illustrates an exemplary parallel resonantcircuit directly connected to a power source in accordance with a secondembodiment the present invention;

[0049]FIG. 4B schematically illustrates the intrinsic or inherentcharacteristics of parallel-connected capacitors of FIG. 4A inaccordance with the present invention;

[0050]FIG. 4C schematically illustrates the intrinsic or inherentcharacteristics of parallel connected capacitors of FIG. 4A forfrequencies far removed from the resonant frequency in accordance withthe present invention;

[0051]FIG. 4D is an exemplary graphical illustration of impedance vs.frequency for the circuit of FIG. 4A in accordance with the presentinvention;

[0052]FIG. 5 schematically illustrates an exemplary parallel resonantcircuit directly connected to a power source in accordance with a thirdembodiment of the present invention;

[0053]FIG. 6 schematically illustrates an exemplary parallel resonantcircuit and its connection within a power system as a stand-alone devicein a filter box form factor configuration in accordance with the presentinvention;

[0054]FIG. 7 schematically illustrates an exemplary parallel resonantcircuit connected directly to a power source some where along thecircuit power line in accordance with the present invention;

[0055]FIG. 8 schematically illustrates exemplary parallel resonantcircuits connected in a three-phase delta configuration power system inaccordance with the present invention;

[0056]FIG. 9 schematically illustrates exemplary parallel resonantcircuits connected in a three-phase wye configuration power system inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0057]FIG. 3A is an exemplary graphical illustration of a voltage 26across a load 4 and a current 28 through it for a circuit shown in FIG.3B in accordance with the present invention. The illustrated circuitthat is schematically shown in FIG. 3B is a power system comprised of apower source 2 coupled directly to a parallel resonant circuit 27 withno intervening components. The parallel resonant circuit 27 is comprisedof an inductor 25 connected in parallel to a capacitor 29, with theresulting circuit 27 connected in parallel to a load 4. The power source2 provides a sinusoidal power signal to the load 4.

[0058] The reactive impedance of the inductor 25 and the capacitor 29 ofparallel resonant circuit 27 are set substantially equal in value, butopposite in sign. Accordingly, they are tuned to resonate at afrequency. Although the parallel resonant circuit 27 may be tuned tooperate at any resonant frequency value (depending on the size of thecomponents), the preferred embodiment is to tune the circuit 27 tooperate at a resonant frequency that matches the operating fundamentalfrequency of the power source 2 to filter out frequencies above or belowthe fundamental, providing low noise signals to load 4.

[0059] For the schematic circuit illustrated in FIG. 3B, the totalimpedance of the path to ground for input signals operating far removedfrom the fundamental includes only that presented by the parallelresonant circuit 27. Therefore, the total impedance of a path to groundfor signals with undesirable operating frequency will behave as ashorted path when the parallel resonant circuit 27 behaves ideally andpresents a “short circuit” behavior. Unlike the prior art, the circuittopography of the present invention has no components that will continueto maintain and present impedance commensurate with their rated values,even when a short path is presented by the parallel resonant circuit.Accordingly, the true impedance of the circuit path to ground for FIG.3B is given by:

Z _(TOTAL) =Z _(PRC)  (7)

[0060] Where

[0061] Z_(PRC)=Impedance of the Parallel Resonant Circuit

[0062] Z_(TOTAL)=Total impedance.

[0063]FIG. 3C graphically illustrates the impedance versus frequency forthe circuit topography schematically illustrated in FIG. 3B, with nointervening components coupled between the parallel resonant circuit 27and the power source 2. As shown, as the frequency increases (moves awayfrom the resonant frequency), the total impedance Z_(TOTAL) (for thepath to ground for these signals) illustrated by line 30 decreases tothereby allow a short path to ground for current signals withundesirable frequencies, filtering out these signals. For largefrequencies, the Z_(TOTAL) will approximately equal zero, as isillustrated in the region 31 of the graph. The main reason for thisregion 31 is due to the intrinsic or inherent impedance values of theparallel resonant circuit 27. No matter how low of an impedancepresented by this circuit, the circuit is still comprised of electricalcomponents (inductor 25 and capacitor 29) that like all others have aninherent or intrinsic impedance values. Hence, for frequencies muchhigher than those desired the impedance presented by the parallelresonant circuit would be approximately zero, with Z_(TOTAL)≈0.Accordingly, most of the undesirable frequencies are filtered with theexception of those with very high frequencies illustrated in region 31.

[0064] Referring back to FIG. 3A, by not coupling any interveningcomponents between the power source 2 and the parallel resonant circuit27, the sinusoidal supply of voltage 26 to the load 4 improves comparedto the prior art FIG. 2A of the present invention. The current 28 drawnby the load 4 at the voltage peak 26 is no longer distorted, and thepeak of the voltage 26 is more pronounced. Given that there are nointervening components, the resonant circuit 27 can now deliver enoughpower at the peak of voltage 26 (where the load 4 draws most of thecurrent 28) to compensate and restore for any signal distortions. Theresonant circuit 27 operating at the fundamental frequency of the powersource 2, through inductor 25 supplies current back into the system torestore any possible distortions of the supply voltage wave form 26during current draw by the load 4. This timing is possible because theresonance of circuit 27 is tuned to resonate at a frequency equal to thefundamental frequency of the power source 2.

[0065] As illustrated, the current 28 drawn at the peak of voltage 26has a narrower horizontal base width with respect to time T, making itvertically more pronounced compared to the prior art FIG. 2A of thepresent invention. In addition, this narrowing of the current 28 at itsbase translates into correction of the voltage waveform 26, making thevoltage 26 more pronounced at the peak. This supply of correct voltage26 and current 28 to the load 4 is possible because the circuit 27 nowfreely supplies these signals without any hindrance or impedance causedby any intervening element, as was the case for the prior art.

[0066]FIG. 4A schematically illustrates an exemplary parallel resonantcircuit 32 directly connected in parallel to a power source 2 with nointervening elements in accordance with a second embodiment of thepresent invention. The parallel resonant circuit 32 is comprised ofthree parallel-connected capacitors 36, 38, and 40 connected in parallelwith a single inductor 34. The capacitive values of each capacitor 36,38, and 40 may be set to be equal or scaled down in size from thehighest to the lowest. The reactive impedance of the inductor 34 and thecombined reactive impedance of the three capacitors 36, 38, and 40 ofparallel resonant circuit 32 are set substantially equal in value, butopposite in sign. Accordingly, the components 34, 36, 38, and 40 aretuned to resonate at a frequency. Although the parallel resonant circuit32 may be tuned to operate at any resonant frequency value (depending onthe size of the components), the preferred embodiment is to tune thecircuit 32 to operate at a resonant frequency that matches the operatingfundamental frequency of the power source 2 to filter out frequenciesabove or below the fundamental, providing low noise signals to load 4.

[0067] Similar to the exemplary circuit shown in FIG. 3B, for theschematically illustrated circuit shown in FIG. 4A the total impedanceof the path to ground for input signals operating far removed from thefundamental includes only that presented by the parallel resonantcircuit 32. Therefore, the total impedance of a path to ground forsignals with undesirable operating frequency will behave as a shortedpath when the parallel resonant circuit 32 behaves ideally and presentsa “short circuit” behavior. Accordingly, the impedance of the circuitpath to ground for FIG. 4A is also given by:

Z _(TOTAL) =Z _(PRC)  (8)

[0068] Where

[0069] Z_(PRC)=Impedance of the Parallel Resonant Circuit

[0070] Z_(TOTAL)=Total impedance.

[0071] The parallel method of coupling capacitors further contributes toattenuation of undesired signals with even higher frequency levelsbecause the parallel combination of these capacitors lowers theiroverall intrinsic or inherent DC resistance R_(CT). FIG. 4B illustratesthe non-idealized view of capacitors 36, 38, and 40 with theirrespective inherent impedance comprised of resistor R_(C36) and inductorL_(C36), resistor R_(C38) and inductor L_(C38), and resistor R_(C40) andinductor L_(C40). As discussed above, at resonance, input signals withfrequencies becoming far removed from the resonance frequency of theparallel resonant circuit 32 (which operates at the fundamental of thepower source 2) see an ever decreasing impedance presented by thecircuit 32. In other words, as illustrated in FIG. 4C, the capacitors36, 38, and 40 behave like a short circuit with the exception of theirintrinsic or inherent impedance. This effectively causes the inherentimpedance of these capacitors to form a parallel connection. However,connecting any resistances (or impedance) in parallel reduces the totalresistance of a circuit. As an example, simple application of Ohms lawusing Kirckoffs Voltage or Current Laws (KVL/KCL) on a circuittopography with two parallel connected resistors (impedance) will showthat for any two impedance with resistances R₁ and R₂, their parallelcombination will have a total resistance value R_(T) that is always lessthan the smallest branch resistance, R₁ or R₂. $\begin{matrix}{R_{T} = {\frac{R_{1} \times R_{2}}{R_{1} + R_{2}} < {{smaller}\quad {of}\quad R_{1}\quad {or}\quad R_{2}}}} & (9)\end{matrix}$

[0072] Or in general, $\begin{matrix}{\frac{1}{R_{T}} = {\frac{1}{R_{1}} + \frac{1}{R_{2}} + \frac{1}{R_{3}} + \frac{1}{R_{N}}}} & (10)\end{matrix}$

[0073] Therefore, the three combined parallel capacitors will have lowerintrinsic or inherent impedance than a single capacitor, contributing tolower total inherent impedance Z_(TOTAL). Application of this concept tothe circuit topography of FIG. 4C will therefore result in attenuationof even higher frequencies that are further removed from the fundamentaldue to these lower inherent impedance values.

[0074]FIG. 4D graphically illustrates the impedance versus frequency forthe circuit topography schematically illustrated in FIG. 4A, with nointervening components between the parallel resonant circuit 32 and thepower source 2. As shown, as the frequency increases (moves away fromthe resonant frequency), the total impedance Z_(TOTAL) (for the path toground for these signals) illustrated by line 42 decreases to therebyallow a short path to ground for current signals with undesirablefrequencies, filtering out these signals. For larger frequencies ofinterest, the Z_(TOTAL) will equal zero. The main reason for thedifference between this graph and the existence of region 31 illustratedin the graph of FIG. 3C is the intrinsic or inherent impedance values ofthe parallel resonant circuit. The parallel combination of thecapacitors 36, 38, and 40 reduced their inherent or intrinsic impedancevalues. Hence, even for frequencies much higher than those desired, theimpedance presented will be negligible, and parallel resonant circuit 32will have zero impedance for most purposes such that Z_(TOTAL)≈0.Z_(TOTAL) will therefore present a short circuit path for signals withfrequencies far removed from the desired operating frequency.

[0075]FIG. 5 is a third embodiment of the power system schematicallyillustrating an exemplary parallel resonant circuit 50 directlyconnected to a power source 2 in accordance with the present invention.The purpose of this circuit is to show that any number of capacitors andinductors may be coupled in parallel to form a resonant circuit. Thecombined reactive impedance of the inductors and the combined reactiveimpedance of the capacitors of parallel resonant circuit 50 are setsubstantially equal in value, but opposite in sign. Accordingly, thecomponents are tuned to resonate at a frequency. Although the parallelresonant circuit 50 may be tuned to operate at any resonant frequencyvalue (depending on the size of the components), the preferredembodiment is to tune the circuit 50 to operate at a resonant frequencythat matches the operating fundamental frequency of the power source 2to filter out frequencies above or below the fundamental, providing lownoise signals to load 4.

[0076]FIG. 6 illustrates the parallel resonant circuit 50 and itsconnection within a power system as a stand-alone device in accordancewith the present invention. As illustrated, the parallel resonantcircuit 50 may be placed in a filter box 52, directly coupled inparallel to a power source 2 and a load 4. FIG. 7 illustrates aschematic drawing of the parallel resonant circuit 50 connected directlyto a power source 2 some where along the circuit power line. Elements54, 56, and 58 are loads that connect to the same power line.

[0077] The physical distance between the parallel resonant circuit 50(within a box as stand-alone or otherwise) and the load 4 or the powersource 2 affects the overall performance of the parallel resonantcircuit. Accordingly, depending on how far away the parallel resonantcircuit 50 is from the load 4 or the power source 2, the level offrequencies that circuit 50 is able to attenuate diminish as thisdistance increases. One reason for this is because the longer the cableor power line connecting the parallel resonant circuit 50 with the load4 or the power source 2, the higher the cable or power line intrinsic orinherent inductive impedance. The cable or the power line present aninductive characteristic, and behave similar to prior art inductors thatwere actually coupled to power lines or cables in series with the powersource or the loads. Therefore, depending on the level of frequencydesired to be filtered, the physical length of the cable or power lineconnecting the resonant circuit 50 with the power source 2 or the loadsshould be taken into consideration and adjusted accordingly.

[0078]FIG. 8 is schematic illustration of the parallel resonant circuits50 connected in a three-phase delta configuration power system inaccordance with the present invention. Three identical parallel resonantcircuits 50, each comprising one or more inductors and one or morecapacitors connected in parallel are constructed. The parallel resonantcircuits 50 are connected between conductor 1, 3, and 5, and in parallelwith the source with no intervening components.

[0079]FIG. 9 is schematic illustration of parallel resonant circuits 50connected in a three-phase wye configuration power system in accordancewith the present invention. Three identical parallel resonant circuits50, each comprising one or more inductors and one or more capacitorsconnected in parallel are constructed. The parallel resonant circuits 50are connected within power line conductors 7, 9, and 11, and in parallelwith the power source with no intervening components.

[0080] While illustrative embodiments of the invention have beendescribed, numerous variations and alternative embodiments will occur tothose skilled in the art. For example, the overall power system and thesensitivity of the load to frequency and signal distortion will dictatethe number, type, and size of the capacitors and inductors used fordesign and engineering of a parallel resonant circuit. Such variationsand alternate embodiments are contemplated, and can be made withoutdeparting from the spirit and the scope of the invention.

What is claimed is:
 1. A filter circuit, comprising: At least one inductor coupled in parallel with at least one capacitor forming a filtering circuit; said filtering circuit directly coupled in parallel with a power source with no intervening electrical components, and tuned to resonate at a frequency equal to a fundamental frequency of said power source, thereby attenuating most other frequencies on said power line and minimizing power dissipation through said filtering circuit.
 2. A filter as claimed in claim 1, where said filtering circuit is a stand-alone device.
 3. A filter as claimed in claim 1, where said filtering circuit is part of a power inlet system.
 4. A filter as claimed in claim 1, where said power source is single phase.
 5. A filter as claimed in claim 1, where said power source is three-phase.
 6. A circuit for correcting perturbations in a power system signal operating at a system line frequency said circuit comprising: at least one capacitive element for drawing a capacitive current; at least one inductive element for drawing an inductive current substantially equal in amplitude and substantially one hundred eighty degrees out of phase with said capacitive current; said at least one inductive element connected in parallel with said at least one capacitive element to form a parallel resonant circuit; said parallel resonant circuit is tuned to resonate at said system line frequency and is connected in parallel directly across a power source with no intervening electrical components between said power source and said parallel resonant circuit; said parallel resonant circuit has circulating currents of substantially the same amplitude as a load current, wherein said parallel resonant circuit absorbs voltage perturbations in excess of the amplitude of said power system signal at all frequencies above and below said system line frequency and wherein said parallel resonant circuit provides energy to restore notches in the amplitude of said power system signal at all frequencies above and below said system line frequency.
 7. A circuit as claimed in claim 6, where said parallel resonant circuit is a stand-alone device.
 8. A circuit as claimed in claim 6, where said parallel resonant circuit is part of a power inlet system.
 9. A circuit as claimed in claim 6, where said power system is a single phase unit.
 10. A circuit as claimed in claim 6, where said power system is a three-phase unit.
 11. A filtering circuit, comprising: a plurality of capacitive elements coupled in parallel; at least one inductor connected in parallel to said plurality of capacitive elements to form a filter circuit; a reactive impedance of said at least one inductor and a combined reactive impedance of said plurality of capacitive elements set substantially equal in value and one hundred and eighty degrees out of phase and tuned to resonate at a frequency value equal to a fundamental frequency value of a parallel connected power source with no intervening components connected between said power source and said filtering circuit.
 12. A circuit as claimed in claim 11, where said filtering circuit is a stand-alone device.
 13. A circuit as claimed in claim 11, where said filtering circuit is part of a power inlet system.
 14. A circuit as claimed in claim 11, where said power system is a single phase unit.
 15. A circuit as claimed in claim 11, where said power system is a three-phase unit. 